Molecular compartmentalisation is a central pillar of life [1]. Demixing of proteins and nucleic acids from solution via physical processes such as liquid-liquid phase separation and surface wetting has been identified as a key mechanism underlying the formation of dynamic membraneless compartments in the cell [2, 3]. Biomolecular condensates, including nucleoli, stress granules, paraspeckles and transcriptional hubs, facilitate concentration of specific molecules at specific sites in response to specific environmental conditions.
Drosophila Behaviour Human Splicing (DBHS) proteins are abundant nucleic acid-binding proteins, involved in transcription, RNA processing, DNA repair and formation of nuclear biomolecular condensates called paraspeckles. While the three DBHS paralogues in vertebrates have a conserved core folded dimerisation domain, their extended intrinsically disordered regions (IDRs) at each terminus are diverse in length and sequence composition.
Using small-angle scattering techniques, X-ray crystallography, in vitro biophysical assays and protein localisation studies in cells, we aim to characterise both folded and disordered regions to describe relationships between amino acid composition, protein structure and material state. DBHS proteins form both homo- and heterodimers, with a preference for heterodimerisation [4]. We hypothesise that differential dimerisation of DBHS proteins provides an avenue for tuning condensate formation and material properties in the cell and provides a unique opportunity for investigation of IDR composition-function relationships. Importantly, we show that the IDRs of these proteins are central to their ability to undergo liquid demixing by phase separation in vitro, and drive formation of dynamic functional condensates in the cell nucleus [5, 6]. Our work indicates that the condensation behaviour conferred by a given IDR on a protein is dependent on the composition of other regions present in the protein. Unexpectedly, we found that the longest LCR found in DBHS proteins, which has a prion-like composition, attenuates protein condensation propensity, adding nuance to the emerging paradigm that IDRs drive protein condensation in the cell. We propose that competing intra- and intermolecular interactions between distinct IDRs provides a mechanism to dynamically regulate the emergent material state of the protein. Further experiments aimed at dissecting IDR sequence elements that mediate these interactions are ongoing.